The proposed studies are designed to determine the causes for and the significance of the extremely high plasma total and protein bound triiodothyronine (T3) concentration in the winter woodchuck. The relationship between the high food intake and increasing body weight of the woodchuck during the summer months and the development of the high plasma T3 concentration will be studied, as well as possible alterations in T3 degradation, the hepatic uptake of thyroxine (T4) and the hepatic conversion of T4 to T3. Seasonal changes in the binding capacities and the binding affinities of the plasma thyroid hormone binding proteins will be studied by vertical slab gel electrophoresis. It will be determined whether plasma T3 concentration alone is elevated or whether the same changes are also seen in the concentrations of the androgens, estrogens and glucocorticoids. The plasma concentrations of protein bound and free T4 and T3 (in intact and thyroidectomized animals) and the conversion of T4 to T3, the movement of T4 and T3 between plasma and the intracellular space and the activities of certain enzymes known to be influenced by T3 (in intact animals) will be measured during the spontaneous periodic arousals from hibernation. In this way, it will be determined whether the high plasma T3 concentration in the winter woodchuck plays a necessary role during the spontaneous periodic arousals from hibernation to this animal.